The present invention relates to a pervaporation method for the separation of liquids in mixture to the respective components or enrichment of one component relative to the other. More particularly, the invention relates to a pervaporation method for the separation of liquids in mixture by use of a pervaporation membrane of a specfic polymeric material with high efficiency.
The pervaporation is a recently developed process for the separation of liquids in a homogeneous mixture into the respective component liquids in which the homogeneous liquid phase of the mixture is partitioned from the vapor phase with a membrane of a polymeric material and a pressure difference is produced between the liquid and the vapor phases, the pressure being higher in the liquid phase than in the vapor phase, or a carrier gas is passed through the vapor phase so that one of the component liquids in the mixture selectively or preferentially permeates through the membrane and is transferred into the vapor phase to cause separation of the components. This method of pervaporation using a polymeric membrane is promising and highlighted as a method for the separation or enrichment of liquids in mixture which can be separated with extreme difficulties in a simple method of conventional chemical technology such as an azeotropic mixture or a mixture of liquids having boiling points close to each other or when the liquid mixture contains one or more of components susceptible to denaturation or degradation by heating.
The efficiency of the pervaporation method for liquid separation largely depends on the performance of the polymeric membrane so that various kinds of polymeric materials have been proposed for the polymeric membrane used in the pervaporation method. Examples of the polymers hitherto proposed for the purpose include cellulose and cellulose derivatives, polyethylene, polypropylene, polystyrene, polyacrylonitrile, polyamide, polyimide, polyester, poly(tetrafluoroethylene), organopolysiloxane and the like as well as various kinds of related copolymers.
The efficiency of a polymeric membrane in the pervaporation is evaluated practically in two respects. Namely, the first of the criteria is the separation factor which is the ratio of enrichment obtained when the liquid mixture permeates through the polymeric membrane and the second is the velocity at which a liquid mixture permeates through the polymeric membrane. Needless to say, a polymeric membrane with larger values of the separation factor and the permeation velocity gives a higher efficiency of liquid separation. When the separation factor of a membrane for the liquids to be separated is small, a sufficiently high degree of enrichment is obtained only by use of the membranes in a multistage sequence. When the permeation velocity of a liquid through the membrane is small, on the other hand, separation of liquids in quantity requires a large area of the membrane or an extremely small thickness of the membrane so that disadvantages are unavoidable in the increased investment for the liquid separation plant or in the insufficient mechanical strength and durability of the polymeric membranes.
The separation factor of a first substance A relative to a second substance B, i.e. .alpha..sub.A-B, by pervaporation is given by the equation .alpha..sub.A-B =(YA/YB)/(XA/XB), in which XA and XB are the weight fractions of the substances A and B, respectively, in the mixture before pervaporation and YA and YB are the weight fractions of the substances A and B, respectively, in the mixture after pervaporation through the membrane.
Taking an example in the separation of ethyl alcohol and water from their mixture by the pervaporation method, the separation factor .alpha..sub.EtOH-H.sbsb.2.sub.O, i.e. the separation factor of ethyl alcohol relative to water, is smaller than 1 in the membranes of cellulose acetate and polyphenylene oxide so that the concentration of ethyl alcohol relative to water is small in the vapor phase than in the liquid phase while the separation factor is larger than 1 in a membrane of an organopolysiloxane so that the concentration of ethyl alcohol is larger in the vapor phase than in the liquid phase. When enrichment of ethyl alcohol in the vapor phase is desired using a membrane of an organopolysiloxane, however, no practically sufficient velocity of pervaporation can be obtained because a thin but strong membrane of unsupported organopolysiloxane can hardly be prepared due to the poor mechanical properties thereof. Nevertheless, no better polymeric materials than organopolysiloxanes are known as a material of pervaporation membrane when enrichment of ethyl alcohol relative to water is desired in the vapor phase.